In gas chromatographs, as well as other apparatus, it is important to control the flow of gas through a load. Some systems operate in response to means for sensing the mass flow through the load, and others operate in response to the pressure applied to it. Mechanical systems are generally inaccurate, have a low degree of reproducibility, and because they do not employ feedback, they are susceptible to variations in temperature as well as to variations in the pressure of the supply gas. Systems are available that utilize a thermal mass flow sensor as a feedback element in a closed loop system so as to provide accuracy in the presence of temperature variations, but their performance at low flow rates reveals a low degree of reproducibility, and their nonlinearity limits the dynamic range within which they can be used with reasonably good results. Furthermore, it is necessary to change the calibration for each gas because of differences in thermal conductivity and specific heat.